This invention relates to removing oxygenates from an olefin stream. In particular, this invention relates to removing oxygenates using extractive distillation.
Olefins, particularly light olefins such as ethylene and propylene, have been traditionally produced from petroleum feedstocks by either catalytic or steam cracking. Oxygenates, however, are becoming an alternative feedstock for making light olefins.
The process of producing olefins from an oxygenate source instead of a higher hydrocarbon source, as in catalytic or steam cracking, can create a variety of problems concerning certain oxygenated contaminates in the olefin product stream. The task of removing these oxygenated contaminates from an olefin containing product stream is a unique and difficult task.
U.S. Pat. No. 6,121,504 discloses a process for directly quenching a gas phase product stream from an oxygenate to olefin process. The direct quenching process causes higher boiling components, such as water and unreacted oxygenate feedstock, to condense and form a heavy product fraction. The heavy product fraction is separated from the lighter olefin product fraction, and the olefin product fraction can be further processed.
Although processes such as direct quenching processes can remove a large percentage of oxygenated components from the light olefin product of an oxygenate to olefin process, a certain amount of oxygenate can nevertheless remain with recovered olefin product. If it is desired to further react the recovered olefins to produce derivative products, the oxygenate contaminants can be problematic in such reaction processes. For example, if it is desired to produce a polyolefin from the olefin product, certain oxygenates may poison the polyolefin catalysts used to make the polyolefin at levels as low as low as 1 ppm. Therefore, it is desirable to remove oxygenate contaminants from olefin containing streams to levels lower than conventional quenching processes can provide.
This invention provides the ability to remove substantially all oxygenate contaminants from an olefin containing stream using extractive distillation. The resulting purified olefin is particularly suitable for use as feed in the manufacture of polyolefins.
Specifically, the invention provides a method of extracting an oxygenate from an olefin containing stream. The method comprises contacting the olefin containing stream with an extractant; and separating the contacted olefin containing stream and extractant using extractive distillation. The olefin containing stream most suitable for the process comprises at least 50 wt. % olefins and not greater than 20 wt. % oxygenates prior to contact with the extractant. Preferably, the extractant is a polar liquid composition at 1 atm., having an average boiling point of at least 100xc2x0 F. (38xc2x0 C.) at 1 atm. More preferably, the polar liquid composition comprises at least 75 wt. % water, alcohol, or a mixture thereof.
The extractive distillation process is particularly effective when operating at pressures higher than conventional quench type processes. Desirably the extractive distillation is carried out at a pressure of from about 100 psia to about 5000 psia.
The invention further provides a method of purifying an olefin containing stream. The method comprises contacting the olefin containing stream with a water containing stream in a first vessel; recovering olefin product from the first vessel; and contacting the olefin product from the first vessel with an extractant in a second vessel at a temperature and pressure higher than that of the first vessel. This embodiment is particularly effective when the olefin containing stream, prior to contact with the extractant, comprises at least 50 wt. % ethylene and propylene, and the extractant is a polar liquid composition at 1 atm., having an average boiling point of at least 100xc2x0 F. (38xc2x0 C.) at 1 atm.
The invention is also directed to a method of purifying and polymerizing an olefin containing stream. This embodiment comprises contacting the olefin containing stream with a water containing stream in a first vessel; recovering olefin product from the first vessel; contacting the olefin product from the first vessel with an extractant in a second vessel at a temperature and pressure higher than that of the first vessel; recovering olefin product from the second vessel; and polymerizing at least one olefin recovered from the second vessel. It is desirable that the olefin containing stream, prior to contact with the extractant, comprises at least 50 wt. % ethylene and propylene, and the extractant is a polar liquid composition at 1 atm., having an average boiling point of at least 100xc2x0 F. (38xc2x0 C.) at 1 atm.